The present invention relates to a nonvolatile semiconductor memory device and, more particularly, to a nonvolatile semiconductor memory device of a floating-gate type.
One conventional nonvolatile semiconductor memory device of a floating-gate type comprises two gates: a floating-gate which is formed between two impurity diffusion regions within a semiconductor substrate and a control gate which is formed above the floating-gate. In this case, the floating-gate, the control gate and the semiconductor substrate are insulated from each other. The write and erase operation for this device, that is, the charging and discharging operation of the floating-gate thereof, is performed by the tunnel effect or avalanche breakdown. For example, in an n-channel type device, accumulation of electrons in the floating-gate results in a high threshold value, while expulsion of electrons from the floating-gate results in a low threshold value. Such two states correspond to the memory states "1" and "0". In other words, in the case of applying a predetermined potential to the control gate, the off-state and the on-state, which represent the absence and presence of a channel under the floating-gate and within the semiconductor substrate, respectively correspond to the memory states "1" and "0".
However, in the above-mentioned conventional device, since each memory cell requires two impurity diffusion regions, that is, a source region and a drain region, the device is large in size and, accordingly, the integration density thereof is decreased.